The significance of unknown input state estimator in advanced control engineering is immense due to its diverse applications. In the past years, different types of unknown input state estimators, which can estimate states without the knowledge of certain input signals have been proposed. Due to this characteristic, it has become a multi-purpose diagnostic tool in the field of fault diagnosis. In this book, a number of different kinds of unknown input state estimators for both linear and nonlinear systems have been discussed. These estimators are very useful in the fields of robust control and fault diagnosis. As a possible use of these estimators, an unknown input state estimator based component Fault Detection and Isolation (FDI) technique is presented. The FDI method is applied for fault diagnosis of different real time systems like single-link robot arm, half car model, electro-hydraulic actuator, etc to show the efficacy of both the estimators and FDI algorithm. This book will be very much useful for graduate and postgraduate students of Control Engineering. Future research scopes are also outlined, which will help the readers to understand the research problems in this area.